Aspects of the present application relate generally to the use of digital optical devices for image projection. The projection systems include illumination sources, image processing, spatial light modulators, and projection optics. The illumination sources project light onto one or more of the spatial light modulators. The illumination light is incident on the spatial light modulators and is changed by the spatial light modulators based on corresponding frame image data to modulate the light and so form an image, which is transmitted into a projection system and the projection system projects an image on a screen, wall, reverse screen or other display surface for viewing. Video sources provide brightness and/or color and image data for projection by the system. Increasingly, increased resolution in the displayed images are desired in these video projection systems. Recent advances include new higher resolution standards for video content delivered in increasingly higher resolution formats such as ultra-high definition (UHD), 4K display resolution and Ultra HD, which require about 4 times the pixel count used in the prior standard 1080 p format of 1080 pixel rows by 1920 pixel columns. A 4K resolution device doubles both the number of rows of pixels and the number of columns from prior high resolution devices, which results in a pixel count that is approximately 4 times the total number of 1080 p pixels.
The advantage of the increasing number of pixels is the information provided in each viewed frame in the display stream is increased, resulting in increasingly visually sharper images. This is especially noticeable to the human visual system (HVS) in settings where the viewer sits very close to the screen, while in the prior lower resolution standards such as SD and HD, the viewer can sometimes see the “screen door” effect as individual pixel boundaries become visible at certain viewing distances, while for 4K or UHD resolution displays, this effect is much harder to see.
In recent years the use of spatial light modulators to project images in environments such as in home theatre applications, in cinemas and for business projection, religious and education projection settings is increasing. Other applications for digital projection systems include office and home video projection systems, portable video projectors, and the like. For systems using digital micro-mirror device (DMD) technology, a spatial light modulator is implemented using the DMD to form a reflective spatial light modulator (SLM). Alternative SLMs include liquid crystal on silicon (LCoS) and liquid crystal display (LCD). For the purpose of the present application, the term “SLM” includes any spatial light modulator, even though certain examples described herein may illustrate systems using DMDs for use in describing the various applications. The use of DMDs can be used to form digital projection systems referred to as “DLP”, a registered trademark of Texas Instruments. DLP® systems have been developed and are commercially available from Texas Instruments Incorporated, the owner of the present application.
As digital image projection systems continue to improve and advance, increasingly higher resolution in the displayed images is required. One approach to increasing the resolution of a displayed image in a single SLM system is to increase the number of pixel elements (for a DMD, each pixel is provided by an individually addressable movable mirrored surface of a micro electro-mechanical device, or (MEMS)). In order to simply display a UHD image using a single SLM, the SLM device would have to have the requisite number of pixels. Increasing the number of mirrors to a higher resolution results in an increased DMD device die size and results in higher costs of production of the DMD device in a semiconductor fabrication facility, and, a correspondingly lower yield. Further, the optical elements in the illumination system that are used to direct light to the surface of the DMD, and the projection optics used to project the reflected images from the DMD and out of the system for display also have to be improved and thus costs for the optics also increase. Additional complexity and costs can occur due to the need to deal with diffraction that results from the decreased pixel size, for example. All of these factors result in increasing SLM device costs and a higher bill of materials for the projection system when SLM size is increased.
A prior known technique for increasing a perceived image resolution is to use an optical actuator in the projection path. In this approach, the SLM surface projects two display images that are created from the incoming image frames and are split into sub-frames for each one of the displayed high resolution image frames. After displaying a first sub-frame for a portion of a frame display time, an optical actuator positioned in the projection path shifts the position of the SLM frames by an amount less than a pixel distance in the horizontal direction, and a second sub frame is displayed for a second portion of a frame time. By shifting back and forth, the image resolution in the image observed by the viewer is increased over the number of physical mirrors by a factor of 2. In this manner a smaller resolution DMD (for example, ½ the number of mirrors) can be used to produce an image with a visual resolution that appears greater than the resolution obtained simply from the number of mirror elements in the DMD.
Use of the optical actuator in this known prior approach results in an image resolution with an apparent increase in the resolution of the image viewed by the viewer. The two images from the two SLM positions are time interleaved sub-frame images so they are not displayed at the same time but instead, are presented in interleaved time periods, taking advantage of the integration characteristics of the HVS. The illumination available in the system is then also split between the two sub-frames. The inclusion of an optical actuator in the optical path will result in a slight loss in brightness.
U.S. Pat. No. 5,490,009, issued Feb. 6, 1996, and assigned to Texas Instruments Incorporated, the owner of the present application, which is hereby incorporated by reference in its entirety herein, describes a method for increasing resolution using multiple SLMs. In this approach, the image to be displayed is divided into n sub-frames. The sub-frames are displayed simultaneously by projecting the sub-frames onto an image plane from multiple SLMs. The sub-frames are horizontally, or vertically, offset. By superposing the multiple sub-frames, the perceived resolution of the image is approximately twice the resolution (for a two SLM system) as for a single SLM system displaying the same image using the same SLM size.
In this prior known approach, an optical actuator is not required. Each of the SLMs can be commercially available SLMs (for example, DMDs from Texas Instruments Incorporated) and the increase in perceived resolution is thus achieved without the need for additional pixels in the SLM devices, and the system operates using the existing optical components. The image frame to be displayed is sampled and split into sub-frames with a horizontal, or vertical, offset. The two sub-frames are superposed and displayed simultaneously by displaying one sub-frame from one SLM and the other sub-frame from the other SLM at the same time. The resolution perceived by a viewer will be doubled in either a horizontal or vertical direction.
In another prior known approach, two SLMs are used with an offset in both horizontal and vertical directions. This approach is described in a European Patent Application EP 0790514 A2, published Aug. 20, 1997 and entitled “A method for displaying spatially offset images using spatial light modulator arrays,” which is assigned to Texas Instruments Incorporated, the owner of the present application, and which is hereby incorporated in its entirety herein by reference. In this prior known approach, two spatial light modulators are used to produce an increased resolution display image from a lower resolution video input by projecting offset images that are offset in both the vertical and horizontal directions by a portion of a pixel pitch, such as ½ pixel diagonal. The image data is divided between the two SLMs so that the visible image has twice the resolution of a single SLM which is displaying the same image data.
While each of the prior known approaches has provided some improvement, further improvements are still desirable. A method for further accurately displaying very high resolution images in a projection system using existing SLM technology, while providing the higher resolution images to the viewer without visible artifacts, and without significant added costs, is therefore needed.
A continuing need thus exists to increase the resolution of the displayed image to accurately display high resolution video content, while maintaining or reducing system costs, and without increasing the size of the SLM devices or the optics in the system.